Quick Search:    advanced search

GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help

This Article Abstract Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Trop, J. M. Search for Related Content GeoRef GeoRef Citation

Latest Cretaceous forearc basin development along an accretionary convergent margin: South-central Alaska

¹ Department of Geology, Bucknell University, Moore Avenue, Lewisburg, Pennsylvania 17837, USA

View larger version (54K): [in this window] [in a new window]   Figure 1. (A) Map showing the present distribution of accreted terranes, uppermost Cretaceous sedimentary deposits, accretionary-prism deposits, and Jurassic–Cretaceous magmatic belts in southern Alaska. This study emphasizes uppermost Cretaceous forearc (MB, PB, WB), intra-arc (PC), and retroarc (CB) deposits exposed outboard (south), within, and inboard (north) of coeval continental-arc plutons (black). Coeval accretionary prism deposits are preserved in the Chugach terrane. This paper integrates new data from the Matanuska Valley–Talkeetna Mountains basin (MB) with recent studies from the Cantwell basin (CB), Pass Creek area (PC), and Wrangell Mountains basin (WB). Uppermost Cretaceous and younger strata and magmatic belts formed within a juvenile continental margin composed of the allochthonous Wrangellia composite terrane (WCT) and para-autochthonous Paleozoic–Mesozoic metamorphic rocks (PZ). Abbreviations not explained on map: A—Anchorage; AR-TM—Alaska Range–Talkeetna Mountains magmatic belt; AV—Augustine volcano; BRF—Border Ranges fault; CB—Cantwell basin; CF—Castle Mountain fault; M—McCarthy; PB—Peninsula deposits; PC—Pass Creek strata; TF—Taral Fault; WB—Wrangell Mountains basin; WF—West Fork fault. Modified from Plafker et al. (1994) and Moll-Stalcup et al. (1994). Age data for uppermost Cretaceous plutons are from Magoon et al. (1976), Csejtey et al. (1978, 1992), Winkler (1992), Drake and Layer (2001), Harlan et al. (2003), and Davidson and McPhillips (2007). (B) Cross section showing generalized crustal structure and tectonic elements of the southern Alaska convergent margin. See A for line of section. Note active northward subduction, arc magmatism, and forearc basin subsidence inboard of the Aleutian trench, analogous to the inferred tectonic framework of the Matanuska Valley–Talkeetna Mountains region during latest Cretaceous time. S.L.—sea level. MC—McHugh Complex, pre-Maastrichtian part of accretionary prism. Adapted from Hudson and Magoon (2002).

View larger version (64K): [in this window] [in a new window]   Figure 2. Geologic map of the Matanuska Valley, southern Talkeetna Mountains, and northern Chugach Mountains. See Figure 1A for map location. Dark gray areas (Km) represent Cretaceous sedimentary strata that are the focus of this study and are interpreted as forearc basin deposits. Numbered circles (1–27) show locations of measured stratigraphic sections discussed in text. Geology from Wilson et al. (1998).

View larger version (35K): [in this window] [in a new window]   Figure 3. Simplified measured stratigraphic sections from upper Matanuska Formation. Note concentration of thick-bedded sandstone turbidites (LA3) south of Castle Mountain fault and thick successions of mudstone, thin-bedded turbidites (LA2), and channel-fill deposits (LA6) north of Castle Mountain fault. See text for discussion and Figure 2 for section locations. Ck—Creek; USGS—U.S. Geological Survey.

View larger version (100K): [in this window] [in a new window]   Figure 4. Representative photos of lithofacies 1–7. (A) Concretionary horizons (white arrows, lithofacies 1), gray mudstone (lithofacies 3), and thin-bedded sandstone (black arrows, lithofacies 5) characterize lithofacies association 1. Hammer (lower right, 30 cm) for scale. (B) Ripple-laminated sandstone (black arrows) and mudstone heteroliths (lithofacies 4). (C) Thin-bedded, normal-graded sedimentation units (light colored, lithofacies 5) separated by structureless mudstone (dark colored, lithofacies 3). Person (lower right) for scale. (D) Close-up of thin-bedded, normal-graded sandstone (lithofacies 5) encased in mudstone (M). Note sharp basal contact, normal grading from coarse (Sc) to medium (Sm) grained sandstone, and Inoceramus shell fragments (arrows). Pen (lower right, 14 cm) for scale. (E) Medium- to thick-bedded sedimentation units (lithofacies 6, 7) that are amalgamated or separated by structureless mudstone (lithofacies 3). Outcrop is 7 m high. (F) Close-up of medium-bedded, normal-graded sedimentation unit (lithofacies 6). Note sharp base (arrow) and upward fining from medium (Sm) to fine-grained (Sf) sandstone, mudstone (M), and shale (Sh). (G) Massive, sheet-shaped thick-bedded sedimentation units (lithofacies 7) separated by dark mudstone (lithofacies 3). Outcrop is 18 m high. (H) Amalgamated thick-bedded sedimentation units (lithofacies 6, 7). Beds dip steeply to the right. Most individual units are amalgamated along discrete contacts (black arrows) or separated by structureless mudstone (lithofacies 3). Most units are tabular, but several units exhibit low-amplitude lenticular geometries (white arrows).

View larger version (95K): [in this window] [in a new window]   Figure 5. Representative photos of sedimentary structures characteristic of lithofacies 5–7 (turbidites), conglomerate of lithofacies 8–9 (debris-flow and/or slurry deposits), and lithofacies association 5 (slump and/or slide deposits). (A) Normal-graded sedimentation unit (lithofacies 6) with massive sandstone (Sm) overlain by horizontally stratified sandstone (Sl). Note sharp base above black shale (Sh, lithofacies 5). Hammer (lower right, 30 cm) for scale. (B) Water-escape structures formed by dewatering of thick-bedded sedimentation unit (lithofacies 7). Vertical to near-vertical pillar columns (white arrows) of structureless to swirled sandstone cross-cut massive sandstone. Teepee structures are present at top of units (black arrows). (C, D) Erosional structures common on bases of medium to thick-bedded sedimentation units (lithofacies 6, 7) include fluted (C) to elongate (D) groove casts. Photograph in C is 40 cm high. Hammer (lower left) in D for scale. (E) Poorly sorted, rounded clasts within a poorly sorted, sandstone-siltstone matrix (lithofacies 9). Pen (13 cm; lower right) for scale. (F) Folded thin-bedded sandstone turbidites (LA3—lithofacies association 3) directly overlie a chaotic zone that consists of sandstone turbidite rafts (olistoliths) and contorted mudstone. Note gently dipping, undistorted strata above and below deformed interval (LA1, LA2—lithofacies associations 1, 2). Olistostromal unit and folded strata are interpreted as the product of sliding and debris flow of previously deposited submarine-slope deposits. Increased abundance and thickness of sandstone turbidites (LA2) directly above deformed horizon is consistent with ponding within accommodation space created via earlier sliding. Outcrop is 35 m high and located 1.2 km southeast of section 20. (G) Close-up of slide-generated folds and olistostrome shown in F. Note outsized sandstone blocks (arrows) in chaotic zone below folds. (H) Contorted sandstone turbidites (white arrows) and mudstone of lithofacies 6 and 3, respectively. Rootless nature of these folds is consistent with deformation via sliding. Outcrop is 16 m high. See section 4 in Figure 2 for location.

View larger version (43K): [in this window] [in a new window]   Figure 6. Maps showing structurally restored paleocurrent data from Campanian–Maastrichtian (A) and Paleocene–Oligocene (B) strata in the Matanuska Valley–Talkeetna Mountains. Detailed reconstruction of Campanian–Maastrichtian sediment dispersal is hampered by the general paucity of unidirectional paleo-current indicators. (A) Sparse south-directed unidirectional data, together with detrital geochronologic ages that match the age of plutons spatially restricted to the north side of the Castle Mountain fault, indicate a component of southward- to southeastward-directed paleoflow. Abundant east-west bidirectional indicators along the center of outcrop belt are interpreted as recording radial spreading of sediment gravity flows on sandy lobes. Postdepositional vertical-axis rotations are also possible, but robust paleomagnetic data are not available from these Campanian–Maastrichtian strata. (B) Paleocurrent indicators from Paleogene alluvial-fluvial strata document south-directed paleoflow along northern basin margin, westward-directed paleoflow along basin axis, and north-directed paleoflow along the southeastern basin margin. Regional late Maastrichtian–Paleocene deformation prompted subaerial uplift of the inboard margin of the accretionary prism. Also note semiradial paleodrainage away from volcanic center related to slab-window magmatism (Cole et al., 2006).

View larger version (12K): [in this window] [in a new window]   Figure 7. Sandstone point-count data from the upper Matanuska Formation plotted on (A) Qm-F-Lt, (B) Q-F-L, (C) Qm-P-K, and (D) Lm-Lv-Ls ternary diagrams. Q—monocrystalline and polycrystalline quartz; F—plagioclase and potassium feldspar; L—lithics, Qm—monocrystalline quartz; Lt—total lithics, including polycrystalline quartz; P—plagioclase feldspar; K—potassium feldspar; Lm—metamorphic lithics; Lv—volcanic lithics; and Ls—sedimentary lithics. Note that Matanuska Formation sandstone (black squares, this study, n = 36) overlaps the dissected arc provenance field of Dickinson et al. (1983) on Qm-F-Lt and Q-F-L plots. Enrichment of plagioclase feldspar relative to potassium feldspar on Qm-P-K plot is also consistent with an arc provenance. Also note variation in Q-F-L plot between forearc and retroarc sandstone. Forearc sandstone includes Matanuska Formation (black squares) and MacColl Ridge Formation (MR, n = 40, Trop et al., 1999). Retroarc sandstone includes northern Cantwell Formation (NC, n = 44; Trop and Ridgway, 1997), southern Cantwell Formation (SC, n = 42, Trop and Ridgway, 1997), and Caribou Pass Formation (CP, n = 22, Hampton et al., 2007). Gray polygons represent one standard deviation from mean modal composition (solid circles). Gray triangles represent mean modal composition of age-equivalent accretionary prism deposits (Dumoulin, 1987). See text for discussion.

View larger version (22K): [in this window] [in a new window]   Figure 8. Geochronologic data for detrital zircon grains from sandstone of the upper Matanuska Formation. Sample represents lower half of sharp-based, normal-graded sandstone turbidite. See section 1 in Figure 3 for sample location. Analytical techniques, age data, and detailed stratigraphic section are in GSA Data Repository (see footnote 1). (A) Age probability histogram showing distribution of U-Pb age determinations for 82 detrital zircon grains. Age determinations represent individual spot analyses from separate detrital zircon grains. U-Pb ages are plotted as histograms with a normalized relative-probability distribution (Ludwig, 2003). Histograms represent 10 m.y. intervals. Relative heights of peaks correspond to statistical significance. Inset shows details of dominant age population. Black bars above plot represent age range of granitic plutons from the Wrangellia and Yukon composite terranes (YTC) in south-central Alaska. Abbreviations: AR-TM—Alaska Range–Talkeetna Mountains belt; CA—Chisana arc; CH—Chitina arc; TA—Talkeetna arc; TB—Taylor Mountains batholith; SA—Skolai arc. Unlabeled black bars represent unnamed plutonic belts. SHRIMP—sensitive high-resolution ion microprobe. (B) U/Th of spot analyses of detrital zircons from sandstone (squares) and granitic clasts (circles).

View larger version (16K): [in this window] [in a new window]   Figure 9. Age histograms of zircon ages from granitoid clasts from conglomerate of the upper Matanuska Formation. Plots show individual spot analyses from separate zircon grains from sections 1 (A–C) and 4 (D–F). Error ellipses are 2. Analytical techniques, age data, and detailed stratigraphic sections are in GSA Data Repository (see footnote 1).

View larger version (56K): [in this window] [in a new window]   Figure 10. Sketch maps (A, B) and cross section (C) showing inferred depositional and tectonic framework of the southern Alaska convergent margin during Campanian–Maastrichtian time. (B) Depositional systems of the upper Matanuska Formation based on this study. See A for location. Locations of measured sections shown in Figure 3 are marked 1–26. Reconstruction includes restoration of 35 km of latest Cretaceous–Cenozoic dextral slip along the southern segment of the Castle Mountain fault. Compare section positions in B and Figure 2. A and C are adapted from Trop and Ridg-way (2007). UKB—uplifted Kahiltna basin; UNB—uplifted Nutzotin basin; UDB—uplifted Dezadeash basin. (D) Explanation of symbols and abbreviations shown in Figures 10A–C.